1. Field of the Invention
The present invention concerns digital multimeters, and in particular hand-held multimeters, which are able to perform a plurality of functions such as measuring voltage, resistance, capacitance and current, and which furthermore include automatic range setting and automatic function setting capabilities.
2. Description of Related Art
European patent document 0 435 431 discloses a state-of-the-art multimeter with automatic function setting for testing respective voltages or resistances of, for example, diodes. A signal recognition circuit is appropriately provided at the input of the multimeter to detect an analog signal, transmit its features in the form of logic signals to a transformation circuit which carries out an appropriate transformation function, and then display the measured, transformed value in response to the analog input signal.
A further development in multimeters with automatic function-setting is described in European patent document 0 487 174, in which a controller is provided for controlling both an A/D converter and a signal-recognition circuit. The apparatus proposed in European patent document 0 487 174 is faster, simpler, and also more economical to manufacture than the above described state-of-the-art device.
The automatic function multimeters of the state of the art use a comparatively low input resistance to measure both impedances and voltages. In many instances, however, the low-resistance measurement of voltage may be a problem because of loading of the voltage source being measured, which can possibly lead to errors. To minimize such measurement errors, digital multimeters as a rule have an input resistance on the order of 10 Megohms. In such multimeters, the measurement function so far has been set manually using a function switch.